Nowadays, with the objective of downsizing, cost-lowering and multi-functioning of optical components, research is being done more and more vigorously concerning optical waveguides and an optical integrated circuit using them. Of the optical waveguides, a polymer optical wave-guide in particular, which is fabricated by coating polymer materials on a substrate, is superior in the mass-productivity and can be expected to be lowered in the cost. Accordingly, in recent years, much attention has been focused thereon.
FIG. 10 shows an embedded type optical waveguide, i.e. an example of conventional polymer optical waveguides. The optical waveguide is fabricated by coating, in turn, a polymer (the refractive index: n1) constituting a lower clad layer 2 and a polymer (the refractive index: n2) constituting a core layer 3 on a substrate 1 such as silicon or glass, and then by coating again a polymer (the refractive index: n3) constituting an upper clad layer 4 after removing an unnecessary portion of the core layer by an etching. In order for the optical waveguide to function as an optical waveguide, n2 is set to be larger than n1, n3. Also, so as to facilitate a connection with an optical fiber as well as to embody a single mode optical waveguide, the polymer material in each layer is selected so that the core layer 3 becomes about 6.times.6.about.9.times.9 .mu.m thick and refractive index differences between the core layer 3 and the clad layers 2, 4 become about 0.3.about.0.7%. Moreover, in order to decrease a propagation loss through an optical waveguide in the above-described polymer optical waveguide, it is necessary to employ polymers of less optical absorption in the lower clad layer 2, the core layer 3 and the upper clad layer 4. In application of an optical fiber communication, it is necessary to lower the waveguide loss for the infrared light (wave length 1.3.about.1.5 .mu.m). However, an ordinary polymer has much CH (carbon-hydrogen) bonds in the structure and has a strong absorption for the infrared light wave because of the absorption by vibration of the CH bonds. Thus, the ordinary polymer is unsuitable for an optical waveguide material for the infrared light. As the optical waveguide material for the infrared light, a polymer in which fluorine (F) or deuterium (D) is substituted for hydrogen (H) in the CH bonds is used. In particular, a fluorinated polymer in which the fluorine substitution is performed has the following excellent properties: For example, the polymer has low loss even for 1.5 .mu.m band and, because the water-repellent characteristic is caused by fluorine, the loss does not increase even when the polymer is used under high humidity. Fluorinated polyimide and fluorinated polymer acrylic, for instance, can be mentioned as the optical fluorinated polymer. Regarding the fluorinated polyimide and a polymer optical waveguide fabricated using it, the description is given in, for example, JP-A-4-9807 and "Electronics Letters", vol. 29, pp. 2107-3109 (1993).
An optical switch taking advantage of a thermo-optical effect, for instance, can be mentioned as an optical integrated circuit using the polymer optical waveguide. Concerning this, the description is given in literatures such as "IEEE Photonics Technology Letters", Vol. 5, pp. 782-784 (1993) and Proceedings of 21-st European Conference on Optical Communication (ECOC'95) 1059-1062 (1995).
Generally speaking, however, because the C--F bonds are very stable, the optical polymer containing fluorine is inferior in an adhesiveness to inorganic materials such as silicon or glass. On account of this, there exists a problem that a delamination occurs between the substrate 1 and the lower clad layer 2 while or after fabricating the optical waveguide. On account of this, there exist problems that it is difficult to fabricate the optical waveguide with the use of the polymer containing fluorine and having the excellent optical characteristics, and that it is impossible to obtain a long-term reliability of the optical waveguide and the optical integrated circuit thus fabricated. As a technique for enhancing the adhesiveness between the polymer containing fluorine and the inorganic base, JP-A-7-174930 discloses a method in which, by coating and baking organic zirconium chelate on the inorganic substrate, organic zirconium compounds are formed and, after that, the fluorinated polymer is coated. However, adhesion strength obtained by using the organic zirconium chelate is not necessarily enough from the view point of the practical utilization. For example, there exists a problem that, if the optical waveguide, after being fabricated by the above-mentioned method, is left under high temperature and high humidity, there occurs a delamination between the substrate and the lower clad layer in much the same way.
In the polymer optical waveguide fabricated by coating the optical polymer on the inorganic base, an object of the present invention is to provide a highly reliable polymer optical waveguide in which the adhesiveness between the lower clad layer and the substrate is enhanced, and a highly reliable optical integrated circuit. Moreover, another object is to provide, by employing them, an optical module and an optical communication system that are excellent at the long-term reliability.
In a polymer optical waveguide which is fabricated on any substrate of a silicon base, a glass base, a silicon base provided with an oxide film on at least a part of the surface thereof, and a substrate having a metallic electrode on at least a part of the surface thereof, and in which a core layer and a clad layer positioned closer to the base than the core layer are made of polymers, a buffer layer made of a polymer is provided between the clad layer positioned closer to the base than the core layer and the base, and a polymer having a strong adhesiveness to the base is employed as the buffer layer. In particular, a polymer containing no fluorine is employed as the buffer layer. Otherwise, in particular, a polymer containing silicon is employed as the buffer layer. Otherwise, in particular, when fluorinated polyimide is employed as the clad layer, polyimide siloxane is employed as the buffer layer. The polymer optical waveguides having the buffer layer are used so as to constitute an optical integrated circuit, an optical switch and an optical module. Furthermore, they are used so as to constitute an optical communication system, thereby making it possible to achieve the above-described objects.